1. Technical Field
The present invention relates to a diode having a plurality of PN junctions.
2. Related Art
JP-B-6-80313 discloses a technique for providing a constant voltage element between a secondary coil and a spark plug of an ignition device. According to this technique, when a discharge voltage of the spark plug reaches a breakdown voltage of the constant voltage element, part of a secondary current flows from the constant voltage element to the ground. Hence, the discharge voltage can be prevented from becoming higher than the breakdown voltage, thereby extending the lifetime of the spark plug.
One example of this kind of constant voltage element is an avalanche diode. However, since the breakdown voltage of the avalanche diode has extremely high temperature dependence, the discharge voltage of the ignition device varies depending on the temperature.
To solve this problem, the inventor studied the use of a diode element 32x, which is included in a semiconductor device of U.S. Pat. No. 5,365,099 (see FIG. 13), as a constant voltage element of a ignition device. This diode 32x has a plurality of N-type regions 32a, 32c and a plurality of P-type regions 32b, 32d. The P-type regions and the N-type regions are alternately arranged in series on an oxide film 41 provided on a surface of a silicon substrate 40 to form PN junctions. Hence, the diode element 32x serves as an equivalent circuit in which a plurality of Zener diodes Da, Db, Dc are connected in series in a state where the adjacent diodes are opposite in direction to each other (see FIG. 5).
There is an inverse relationship between a temperature characteristic of voltage (forward voltage VF) obtained when a voltage is applied in the forward direction of the Zener diodes Da, Db, Dc (see FIG. 6A) and a temperature characteristic of Zener voltage (backward withstand voltage VR) obtained when a voltage is applied in the backward direction of the Zener diodes Da, Db, Dc (see FIG. 6B). That is, as the temperature becomes higher, the forward voltage becomes lower and the backward withstand voltage becomes higher. Hence, a pair of Zener diodes Da, Db, which are connected so as to be opposite in direction to each other and opposed to each other, mutually cancels increase and decrease of the voltage value caused due to the temperature characteristic. Hence, the temperature characteristic of the Zener voltage of the diode element 32x can be moderate.
In the case of the ignition device, an extremely high Zener voltage is required for the constant voltage element. However, the diode element 32x installed in the semiconductor element of U.S. Pat. No. 5,365,099 is not assumed to use such a high Zener voltage. Hence, the present inventor studied increasing the number of PN junctions of the diode element 32x disclosed in U.S. Pat. No. 5,365,099 to make the temperature characteristic moderate while having a high Zener voltage.
Meanwhile, in the semiconductor element of U.S. Pat. No. 5,365,099, the diode element 32x is integrated together with a power MOSFET or an IGBT element illustrated by reference numeral 42 in FIG. 13. Hence, the semiconductor element has a configuration in which the diode element 32x is mounted on a silicon substrate 40 forming various semiconductor elements.
The diode element 32x is formed on a silicon dioxide film 41 and is insulated from the silicon substrate 40. Hence, as studied above, if increasing the number of the PN junctions to increase the Zener voltage of the diode element 32x, the silicon dioxide film 41, which is formed by thermal oxidation of silicon and has a thickness of about 1 μm, is limited to increasing withstand voltage (800 V at the most). Hence, it has found that the structure of the diode element 32x of U.S. Pat. No. 5,365,099 cannot sufficiently increase the Zener voltage.
In addition, if increasing the thickness of the silicon dioxide film 41 to increase the withstand voltage of the silicon dioxide film 41, large stress is generated at an interface of the silicon dioxide film 41 due to thermal expansion difference because thermal expansion coefficients are largely different between the silicon substrate 40 and the silicon dioxide film 41. In addition, it is basically difficult to form an oxide film having a thickness significantly exceeding 1 μm by thermal oxidation.
Note that the above problems similarly occur not only in a constant voltage element provided at the secondary side of the ignition device but also in diodes, for which high withstand voltage is required, such as one provided at the primary side of the ignition device and one provided in a device other than the ignition device.